How to use the roofing calculator
Add a section for each part of the roof: a gable, a hip or a shed. Type its length, its width and its overhang. For a gable, the length runs along the ridge and the width is the span across it; the tail at the eaves and the rake ends is added for you. Type the pitch, such as 6:12, and a waste percentage. The page gives the roof area, the roofing squares and the bundles of shingles, plus the ridge and hip cap, starter, underlayment, ice barrier, drip edge and valley lining, and draws your roof from above with the courses filling in and the bundles counted. Where it goes shows the layers in order, and Metal roof figures panels for the same sections. Nothing you type leaves your device unless you make a share link.
The formula
roofing squares = roof area ÷ 100
When every plane of a roof has the same pitch, its surface area is the area it covers from above times one number, the slope factor. At 6:12 the rise is 6 for every 12 of run, so the factor is √(1 + 0.5²) = 1.118: the roof is about 12 percent bigger than the plan under it. That holds for gables, hips, sheds and cross gables with valleys. A roofing square is 100 square feet of roof. Hips and valleys run along the plan diagonal, so for every foot of common run they are √(2 + (rise ÷ run)²) feet long, which is 1.5 at 6:12.
A worked example
A gable roof is 40 feet long and 28 feet wide, with a 1 foot overhang at the eaves and at the rake ends, at 6:12. From above it covers (40 + 2) × (28 + 2) = 1,260 square feet. Times 1.118 that is 1,408.7 square feet of roof, or 14.09 squares. Add 12 percent for waste and it is 15.8 squares; at 3 bundles a square that is 47.3, so 48 bundles. The ridge is 42 feet, which takes 2 bundles of cap at 25 feet each. The eaves are 84 feet, which takes 1 bundle of starter at about 120 feet. The rakes add 67.1 feet, so the drip edge is 151.1 feet, or 16 pieces of 10 feet. The ice barrier runs from the eave to 24 inches inside the wall line, which is (1 + 2) × 1.118 = 3.35 feet up the slope, over 84 feet of eaves: 281.7 square feet, or 2 rolls of 200. That is the example the page opens with.
Hips, valleys, eaves and rakes
The edges decide the small materials. Eaves are the low edges, rakes are the sloped edges at the ends of a gable, and the ridge and hips take cap shingles. A hip roof over the same 40 by 28 foot house has a 12 foot ridge and four hips of 22.5 feet, 90 feet in all, and no rakes. A valley is where a cross gable meets a larger roof: mark a section as meeting one or two valleys and the page adds their length, assuming equal pitch and a wing no wider than the roof it joins. Dormer roofs go under Add roof surface; skylights and chimneys under Subtract.
Underlayment, ice barrier and drip edge
The underlayment is the roof area not covered by ice barrier. In the 2015 International Residential Code, asphalt shingles are allowed only on slopes of 2:12 or steeper, and slopes from 2:12 up to 4:12 need double underlayment, which the page counts. Where Table R301.2(1) records a history of ice at the eaves, an ice barrier runs from the lowest edge to at least 24 inches inside the exterior wall line, and the 2015 text adds 36 inches up the slope on roofs of 8:12 or steeper; the page uses the larger distance. Drip edge has been required at eaves and rakes since the 2012 edition, under the underlayment at the eaves and over it at the rakes. Editions differ and states amend them, so check yours with your building department.
Waste
Shingles are cut at the ridge, hips and valleys and around vents. The page starts at 12 percent, the same as the shingle job in the AerialQuote map tool; a plain gable needs less and a cut-up roof with hips and valleys more (some trade guides say 15). It is a starting value you can change. Bundles round up to a whole bundle.
Where the numbers come from
A roofing square is a trade unit of 100 square feet. A bundle of most standard and architectural shingles covers about a third of a square, so 3 bundles make a square; some heavier shingles take 4 and a few take 5, so use the number on your wrapper. GAF lists about 25 feet of ridge per bundle of Seal-A-Ridge cap and about 120 feet per bundle of Pro-Start starter; other brands differ. Common rolls cover 1,000 square feet for synthetic underlayment 48 inches by 250 feet, 432 square feet for 15 pound felt 36 inches by 144 feet, and 200 square feet for a self-adhering ice barrier 36 inches by 66 feet 8 inches. The slope and hip factors are geometry, checked against a 3D model of each roof. This page does not replace the code, a roofer or an inspector, and it does not size the roof structure.
Common mistakes
- Measuring the house and forgetting the overhang. The tails add area on every side. Type them.
- Using floor area as roof area. A 6:12 roof is 12 percent bigger than its plan; a 12:12 roof is 41 percent bigger.
- Ordering exact. Bundles come whole, and cuts at ridges and valleys waste some.
- Forgetting the small parts. Starter, cap, drip edge and valley lining are separate items.
- Assuming the same code everywhere. Ice barrier and low-slope rules depend on your location and edition.
Questions
How many bundles are in a square?
Most standard and architectural shingles are 3 bundles per square. Some heavier shingles are 4, and a few are more. Check the wrapper, then pick the number under Materials.
How do I get roof area from my floor plan?
Multiply the plan area, with the overhangs, by the slope factor: 1.118 at 6:12, 1.202 at 8:12, 1.414 at 12:12. The Chart tab lists the factors.
Do I need an ice barrier?
Only where your code says so, usually where ice backs water up under the shingles at the eaves. It is cheap insurance in cold climates, which is why the box starts ticked; untick it if your code does not ask for it.
Can I use metric?
Yes. Press Metric to type meters and centimeters. The page opens in the system your browser's region uses and remembers your choice. Roofing squares and bundle counts stay as they are sold.
Can I share my result?
Yes. Share makes a link with a picture for Facebook, a link you can copy, and a short video for TikTok made on your device. The link keeps only the sections, pitch, waste and bundle count, for 90 days. It never keeps a price.